Faith
Every faith in the Known World came here in somebody's head. The arrivals brought them, across centuries, from a world whose gods they could describe and whose temples they would never see again. Nothing here was founded: no revelation anybody can point to, no prophet who walked in this world, no holy ground that was holy before somebody decided it was. So the world holds a great many faiths and no true one, and has built no machinery for deciding between them.
What the philosophy says
Nothing. It has no position on whether any god exists, and no city has ever been asked to take one. That follows from the two tyrannies the Pact was written against: a faith imposed by force is the first, and a faith imposed by everybody agreeing is the second. What the philosophy asks about a faith is what it asks about everything — have you examined it, is it yours? A person who believes what their parents believed and has never once turned it over has failed the Personal aspect, the same failure as following a trade one never chose. The unexamined belief is the problem; what the belief is, is nobody's business. So a devout man and an unbeliever hold each other to the same standard and are both satisfied.
What the cities require
A faith may do as it likes so long as it does not demand that another person's liberty be violated and does not attempt to impose itself. No faith holds law anywhere; no magistrate asks what a man believes, no office requires it, no oath invokes anything. Proselytizing is forbidden by nobody and enjoyed by nobody, since pressing a belief on somebody is uncomfortably close to the second tyranny. Faiths grow by household and by example.
What came across
Everything — every faith our own world has held, in the form it held when its bearer crossed. The northern cities are thick with faiths from the north and west of our world, Eldershore with those of West and East Africa, Clearhold with those of South and East Asia, and every city with all of it mixed in, because nine centuries of trade moves beliefs as readily as Aruna. Some dwindled to a handful of households; some arrived in one person and died with them. What flourished were the faiths that could live beside the Oathbond without a quarrel — duty, self-mastery, the dead, the turning of the year, living as one's nature requires. What dwindled were the faiths that required a city to enforce them.
What practice looks like
Whatever the faith says: buildings and set days, or a house, or a shelf, a shrine, a name said at a particular hour, a thing done before a meal the children learn without being told why. Funerals are where a faith is most visible, because everybody attends one; a household buries its own, a guild a member, a hall its line, and a city whoever has nobody. What everybody shares is the calendar: the five festival days come from it rather than from any faith, and every faith has laid something of its own over them — Dar most of all.
Who leads
No priesthood as an institution; people who hold roles in particular traditions, paid by the congregation, a household, a guild, or not at all. Stormharbor's Gothi are the most visible — spiritual witness at a Goenidd, presiding at the Skald Hall, speaking at a burial, with real standing in the city. Eldershore's ancestral keepers hold a line's dead in memory and address them directly. Nobody has authority beyond their own believers; a Gothi may say what he likes and a magistrate will listen politely and do as the law requires. On slavery the faiths hold no single doctrine and a broad practical agreement; see Protection and Its Failures#The religions.
Slaves
A slave holds no Oathbond. She holds whatever she believes. What she thinks about the dead or the turning of the year is not property and cannot be transferred with a Bill of Chattel. She practices as her household allows her time to; owners and slaves in one household frequently do not share a faith, and he does not ask her to believe what he believes, which would be the second tyranny inside one house.
The argument nobody wins
Because no faith can be established and none disproved, the argument never resolves and never stops. Shining Reach argues most. Clearhold treats the question as unanswerable and therefore not worth examination time, which is itself a position. Forgegate's workshops hold quiet unbelief and quiet belief and nobody at the bench asks which. Eldershore's ancestors are addressed directly by people who would not call themselves religious. Stormharbor takes it seriously, says so, and would defend your right to disagree while telling you that you are wrong. The Iron Coves have faith and no institutions, and generations with nobody correcting anything have produced versions of old faiths their founders would find hard to follow; crews are devout in the particular way of people whose lives turn on weather.